The invention relates to an improved process for preparing cationic or amphoteric acrylamide (co)polymers. More specifically, the invention relates to preparing cationic or amphoteric acrylamide (co)polymers by a degradation reaction called a Hofmann degradation in aqueous solution on a base (co)polymer and is characterized in that the reaction medium contains a high proportion of polyvalent cationic ions before reagents, i.e. hypohalide and hydroxide, are added.
The examples of cationic acrylamide polymers known in the prior art are of various kinds: most often they are copolymers based upon acrylamide and cationic monomers, which can also be polymers called Mannich polymers (the acrylamide polymer reacts with formaldehyde and a secondary amine then is quaternized), glyoxal polymers or acrylamide polymers on which a Hofmann degradation has been done.
Among all these polymers, only those obtained by a Hofmann degradation reaction have specific properties not shared in terms of cationic charge density.
Hofmann Degradation
This reaction, discovered by Hofmann at the end of the nineteenth century, can change an amide function into a primary amine function with elimination of a carbon atom. The reaction mechanism is given below.
In the presence of a base (e.g.: NaOH), a proton is removed from the amide.
The amidate ion formed then reacts with the active chlorine (Cl2) in hypochlorite (e.g.: NaClO, which is in equilibrium: 2NaOH+Cl2NaOCl+NaCl+H2O) to give an N-chloramide. The base (NaOH) removes a proton from the chloramide to form an anion. The anion loses a chloride ion to form a nitrene, which rearranges into an isocyanate.

The hydroxide ion and the isocyanate react to form a carbamate.R— N═C═Ō+OH−→R—NH—CO2−
After the carbamate is decarboxylated (removal of CO2 in the presence of acid, e.g.: HCl), a primary amine is formed in the form of an acid salt.

To convert all or part of the amide functions of an acrylamide (co)polymer into amine functions, two main factors are involved (expressed in molar ratios). These are: —Alpha=(alkali and/or alkaline earth metal hypohalide/acrylamide) and—Beta=(alkali and/or alkaline earth metal hydroxide/alkali and/or alkaline earth metal hypohalide).
The polymers obtained by Hofmann degradation reaction are in particular used in paper-making processes. Generally, it is known that to be effective as a strength agent, the (co)polymers used must be in the most cationic form possible and therefore have a high degree of degradation. Indeed, as a function of the Alpha degree of degradation, it is possible to generate variations in cationicity related to the number of amine functions produced on the carbon backbone of the (co)polymer. Moreover, it is known that when the product obtained by Hofmann degradation reaction is amphoteric, it can also be used to improve the charge retention when the paper or cardboard is manufactured.
Until recently, only very expensive processes using, in situ, a Hofmann degradation product manufacturing unit (EP 377313) or processes using another polymer (base of the N-vinylformamide (co)polymer type followed by hydrolysis) itself relatively costly (US 2004/118540), had been implemented.
It was not until early 2005 and patent WO2006075115 by the applicant that a viable industrial solution was proposed. The invention there is characterized, in part, by the fact that the Hofmann degradation product claimed is an organic polymer produced at a concentration greater than 3.5% by weight and preferably greater than 4.5%. Even though the dry strength performance is greatly improved, the polymers produced remain unstable over time and have very low molecular weight, giving the (co)polymer very limited utility for applications such as draining or flocculation.
Patent WO2008/107620 by the applicant had resolved this problem of stability and poor performance during dewatering by changing the nature of the hypochlorite (using calcium hypochlorite). However, the inventor then had to handle low calcium hypochlorite availability, with high impurity content in the final product (the purest grade of calcium hypochlorite has a minimum level of impurities of 20%), and storage, transport and handling problems for a powder classed as dangerous.
The problem that the invention proposes solving is therefore to propose an improved process for preparing cationic or amphoteric acrylamide (co)polymers while retaining in said polymers their characteristics of stability and high molecular weight.